The invention relates to electrical prospecting of the subsoil.
Electrical prospecting has been known since the first works of Conrad Schlumberger were published in 1920. On the ground are placed two "ground terminals", or exciter electrodes; the latter are subjected to a continuous current, and this current is propagated through the different layers of the subsoil. The effects of this current are examined with the help of two detecting electrodes, which are moved step by step on the surface of the ground, each time measuring the potential difference.
Generally, the separation of the two detecting electrodes is small in relation to the distance between the two ground terminals. From each potential difference measured, an apparent resistivity of the subsoil is drawn which is associated with the midpoint of a segment joining the two measuring electrodes. By making these measurements at many points in the region of the two exciter electrodes, a map of apparent resistivity is made.
A geophysicist specialist can interpret the resistivity map, often with the help of other information, for example, geophysical information. At the present time, numerical mathematical treatments are used to determine a theoretical resistivity map from a probable model of the subsoil. By the difference between the map obtained on the terrain and the theoretical map, resistivity anomalies representing heterogenies of the subsoil can be obtained.
Subsoil prospecting techniques using alternating current are also known. However, their use is more rare because, compared to continuous current, the alternating currents have the tendency to be propagated less deeply in the soil even though their frequency is greater.
Thus, it is electrical prospecting using continuous current which is preferred. But, even though it reaches deeper layers than prospecting with alternating current, it is still difficult with this method to determine deep anomalies situated below a high resistivity masking layer. Deposits, whether they be petroleum, ore-bearing or geothermal, most often show up as this type of deep anomaly.